This apparatus is directed an amplifier system for use forming output signal of a downhole logging tool. A logging tool forms a signal which is transmitted from a fluid tight sonde supported on an armored logging cable. The logging cable includes one or more conductors in the logging cable. Typically, the signal of interest is transmitted up the logging cable and is obsured by noise picked up in transmission from the sonde to the signal processing equipment at the surface. For instance, a typical logging cable will include a power conductor for providing electrical power to the sonde. Logging cables are quite long, easily as long as 25,000 feet. The noise picked up by a conductor for the signal of interest is substantial. Frequently the sonde operational power and the signals from the sonde share the same two conductors.
Typically, a sonde is lowered to the bottom of a well and then is retrieved from the well pulling the logging cable and sonde up the well as logging is continued. Clearly, there are multiple sources of noise imposed on the signal conductor in a logging cable. Additional noise may occur because a portion of the logging cable is coiled on a spool or reel which tends to act as an inductor, thereby increasing the noise level on the signal conductor because the self inductance of the coiled logging cable adds a signal component into the logging cable. This apparatus is directed to an amplifier system adapted to be located at the surface and connected to the signal conductor for amplification of the signal (with noise) to thereby reduce the relative amplitude of the noise imposed on the signal.